Efforts are being made to reduce emissions of carbon dioxide (CO2) and otherwise purify auto emissions to thereby reduce harmful environmental effects. With respect to diesel engines, for instance, it has been proposed to increase the pressure of injected fuel, inject fuel in multiple stages, and the like to thereby improve emissions. To achieve these purposes, high response and short injection intervals are typically required of the solenoid valves of injectors.
However, conventional solenoid valves suffer from certain disadvantages. For instance, when a magnet coil is energized and to magnetize a stator, an armature is attracted by the stator and moves in fuel at high speed. The armature in conventional solenoid valve meets with the resistance of the fuel (i.e., fluid drag). The fluid drag has an undesirable effect on response.
In partial response to this problem, U.S. Pat. No. 6,648,248 (Japanese Patent Publication No. 2001-304448) discloses a device with passages that establish communication between a valve chamber filled with fuel and a discharge passage of an injector. The passages are provided around an armature. However, the passages are provided in a component other than the solenoid valve. This complicates the construction of the injector, which leads to an increase in cost.
Another technique has been proposed as illustrated in FIG. 7. As shown, notches 110 are formed in the outer circumferential surface of an armature 100. Communication grooves 130 are also included that establish communication between the notches 110 and a central recess 120 formed in the center of the armature 100. Fluid drag produced when the armature 100 is moved in fuel at high speed is thereby reduced.
However, this technique also suffers from certain disadvantages. Specifically, when the armature 100 is formed of highly magnetic material (e.g. silicon steel) to enhance the solenoid response, the strength of the armature 100 is relatively low. (For example, silicon steel has highly magnetic properties but it is a low-strength material). Also, the armature 100 typically includes relatively thin-walled portions, such as between the notches 110 and the central recess 120. Stress concentrations can develop at these thin-walled portions. Therefore, it may be difficult to form the communication grooves 130 having a sufficient passage area in the thin-walled portions of the armature 100 if it is made out of relatively low-strength magnetic material.